A vacuum booster provided on a vehicle brake device includes a negative pressure chamber and a variable pressure chamber connected to an intake pipe of an engine and a vacuum pump. The variable pressure chamber communicates with the negative pressure chamber when the brake operation is not performed, and is opened to atmosphere when the brake operation is performed with the communication with the negative pressure chamber blocked. Therefore, when the brake operation is performed, pressure in the variable pressure chamber is increased by atmospheric air flowing into the variable pressure chamber, and differential pressure between the negative pressure chamber and the variable pressure chamber increases. Then, the vacuum booster utilizes the differential pressure to assist brake operation force being operation force of a brake pedal by the driver.
Incidentally, in a case where the brake operation is performed in a situation in which negative pressure in the negative pressure chamber is low, the above-described differential pressure is less likely to increase. Also, in a case where an operating speed of the brake pedal by the driver is high, that is, in a case where the driver who performs the brake operation requests emergency braking, the pressure in the variable pressure chamber might tend to decrease and the above-described differential pressure might become small. When the above-described differential pressure does not increase in this manner, assisting force by the vacuum booster is less likely to increase.
Therefore, the brake control device disclosed in Patent Literature 1 starts the brake assistance processing for assisting the increase in hydraulic pressure in the wheel cylinder provided for the wheel earlier as the operating speed of the brake pedal is higher. As a result, even in a case where the operating speed of the brake pedal is high and the pressure in the variable pressure chamber is decreased, it is possible to assist the increase in brake force applied to the vehicle before the vacuum booster reaches assistance limit. Meanwhile, the “assistance limit” herein means a state in which the increase in the assisting force by the vacuum booster cannot follow the increase in operation force of the brake pedal by the driver.
Herein, an example of the brake assistance processing performed when the vacuum booster reaches the assistance limit is described. That is, when the brake pedal is operated by the driver, the negative pressure in the negative pressure chamber is monitored, the hydraulic pressure in the master cylinder at a point in time when the vacuum booster reaches the assistance limit is predicted from the negative pressure, and a predicted value is made a negative pressure conversion hydraulic pressure. Meanwhile, the negative pressure conversion hydraulic pressure is calculated using a map prepared in advance, and the negative pressure conversion hydraulic pressure increases as the negative pressure in the negative pressure chamber is higher.
When a start condition of the brake assistance processing is satisfied, the negative pressure conversion hydraulic pressure at that point in time is recognized as assistance limit pressure, and target hydraulic pressure is set on the basis of the assistance limit pressure. For example, target hydraulic pressure F1 may be represented by the following relational expression (equation 1). Meanwhile, in relational expression (equation 1), “Pmc” represents the hydraulic pressure in the master cylinder, “Pmb” represents the assistance limit pressure, and “Gadj” represents a predetermined correction gain. For example, the predetermined correction gain Gadj may be set to a gain when the brake operation ideal in terms of a design of the vacuum booster is performed.[Expression 1]F1=Pmc+Gadj·(Pmc−Pmb)  (Equation 1)
Then, a difference obtained by subtracting a current MC pressure Pmc in the master cylinder from the target hydraulic pressure F1 calculated in this manner becomes the target increase amount, and a brake actuator is operated according to the target increase amount. As a result, the hydraulic pressure in the wheel cylinder may be brought closer to the target hydraulic pressure.